Making fine mesh silica screens



Oct. 23, 1951 H. B. LAW I 2,572,497

MAKING FINE MESH SILICA SCREENS Filed Nov. 30, 1948 INVENTOR H RULD B. LAW

TgNEY Patented Oct. 23, 1951 MAKING FINE MESH SILICA SCREENS Harold Bell Law, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 30, 1948, Serial No. 62,731

This invention relates to the making of insulator electrodes and particularly to the making of fine-mesh silica screens for use in television camera pickup tubes.

One type of television camera pickup tube is that disclosed in U. S. Patent 2,185,609 to Sohroter and in U. S. Patent 2,322,807 to Iams. The television camera pickup tubes disclosed in these two patents employ a fine mesh screen photosensitized on one side and upon which a charge pattern is produced for controlling a flow of electrons through the screen.

The scene to be televised is focussed upon the photosensitized side of the fine mesh screen so that photoemission takes place, leaving on the screen a charge pattern corresponding to the light picture projected onto the screen. The other side of the fine mesh screen may be either scanned by an electron beam or may be subjected to a continuous spray of electrons. In either case, the electrons of the beam or-spray will penetrate through the fine mesh screen to an extent governed by the charge pattern on the photosensitized side of the screen. If an electron beam is used, that portion of the beam penetrating the fine mesh screen is collected by a positive electrode and is amplified by the circuit of the electrode to form a'video signal. If a spray of electrons is used, the portion of the spray passing through the fine mesh screen may be focussed onto a fluorescent screen to produce a visible image corresponding from point to point to the intensity of the spray passing through the screen.

In tubes of the types disclosed by these patents, the fine mesh screen is of a type made by coating a fine metal screen with an insulating material. Usually, in tubes of this type, the metal screen is connected to a source of potential which provides a high capacity between the metal mesh and the outer surface of the insulating coating, upon which the charge image is produced. For some purposes, the high capacity between the surface of the insulating .coating and the metal screen is a disadvantage, due to the inability of producing high differences of potential between the metal screen and the coating surface by photoemission.

In tubes of the type disclosed in-the patents cited above, under certain conditions, it would be desirable to have a fine mesh screen for controlling the electron flow therethrough, which has a low capacity between portions of the screen. A control mesh screen of low capacity would have the advantage that photoemlssion from the 6 Claims. (01. 18-57) screen surface would tend to leave a charge pattern on the screen surface with high differences of potential.

It is, accordingly, an object of my invention to provide an insulator electrode for a television camera pickup tube.

It is an additional purpose of my invention to provide a fine mesh screen made entirely of insulating material. 7

It is another object of my invention to provide a method for making a fine mesh silica screen.

It is another object of my invention to provide a fine mesh silica screen having low capacity for use in television camera pickup tubes.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

Figure l is a cross-sectional view of a fine mesh screen used in forming the novel screen of my invention.

Figure 2 is a cross-sectional view of the fine mesh screen formed according to my invention.

Figure 3 is a partial plan view of a fine mesh screen. made according to my invention.

Figure 4 is a sectional View of a television camera pickup tube utilizing a fine mesh screen according to my invention.

Fine mesh wire screens are used in various types of television camera pickup tubes and storage tubes. These screens are made of metal either by plating or from woven wire. The fineness of the screen may vary between 200 and 1000 mesh per inch.

One successful method of making a fine mesh metal screen is that used for making a screen of 500 mesh or more per inch for a television camera pickup tube This method is described fully in my co-pending application Serial No. 666,037 filed April 30, 1946 which issued as Patent No. 2,529,086, Nov. '7, 195-0. The method involves the use of a glass master having a set of parallel grooves ruled in the surface of a glass plate and spaced from each other a distance required to grooves of the glass plate. Next, by any suitable means, the metal network within the grooves of the glass plate is plated with a relatively heavy layer of copper metal which results in a metallic network within the ruled grooves of the glass plate. The amount of copper plated on the sputtered metal is that which is sufiicient to provide a self-supporting mesh screen which can be stripped from the glass plate and used as is desired.

A fine mesh copper screen II made in any desired manner such as by the method suggested above, is used for my novel method of making a fine mesh silica screen I4. A copper screen is used, having the desired fineness. The copper screen II may be first mounted on a ring of metal (not shown) by cement soldering, welding or in any manner so as to firmly support the screen during processing. The ring of metal with the copper screen I I is brought into contact with a mixture of silicon tetrachloride (S1014) and water vapor which will react chemically to form a precipitation of hydrated silica (SiO2'1LI-I2O) on the copper wire screen. This reaction may be promoted in any desired manner. For example, silicon tetrachloride vapor and water vapor as steam may be introduced into a container at a point adjacent the copper screen. Another method is to place the ring of metal holding the copper screen over the top of a wide-mouthed bottle containing a small quantity of silicon tetrachloride. This assembly is placed in an enclosure or container of some type with a beaker of Water and maintained at room temperature. From time to time, the ring holding the copper screen is turned over. The copper screen is thus exposed to the silicon chloride (SiCl-z) fumes in the presence of the water vapor for a period of time varying as is desired from minutes to 30 minutes. Whichever method is used, will produce a layer I3 of hydrated silica on the copper screen wires.

The coated wire screen H is then placed in concentrated nitric acid for a considerable time, normally several days, to'give the acid a chance tQpenetrate through chance pinholes in the hydrated silica coating, and to react with the copper metal of the screen II,, and to decompose the metal screen II. A fine mesh screen I4 is left which consists of a hollow tubular structure, as shown in Figure 2, for example.

A much quicker method to remove the copper screen II from its silica coating I3, however, is to wash the coated screen in water and then allow it to partially dry before putting it in acid. During the partial drying, the hydrated silica layer I3 begins to dehydrate and shrink. Since the copper does not shrink, cracks 40 will form in the layer I3, which will be sufiicient to allow the acid to penetrate to the wire screen II at many places. Care must be taken to prevent the partial drying from being carried too far, or the silica coating I3 Will become too badly cracked. The coated screen II is then put into concentrated nitric acid and the copper is eaten out or chemically decomposed much more rapidly. After the copper screen II has been completely removed by the acid bath, the resulting silica screen I4 is completely dehydrated bydrying and maintaining the screen fiat during the drying process. The Water driven off from the silica during the drying processes is not regained and the final screen is actually a tubular structure of silicon dioxide, as shown in Figures 2 and'3.

Another method of depositing silica. upon the fine mesh copper screen may be that in which ethyl silicate vapor is caused to decompose at a high temperature and in a manner to precipitate a silica coating I3 on the wire screen II. This method is fully described in my co-pending application Serial No. '734,l56,filedMarch12,1947, which issued as Patent No. 2,540,623, Feb. 6, 1951. The method described in the above cited cO-pending application is that in which a'fine mesh metal screen is heated to substantially 950 C. in an atmosphere of ethyl silicate vapor. The heat of the metal screen causes the metal silicate to dissociate into silica particles which are deposited on the screen as a coating. A silica mesh, formed formed by this method using ethyl silicate vapor, has few if any cracks since dehydration takes place as the silica is deposited at the high temperatures. Also, the screen resulting from the method is quite flat.

The fine mesh silica screen resulting from the above described methods is one havinghigh electrical resistance. Such a screen can be used, for example, in a television camera .pickup tube, of the type shown in Figure 4, in which the silica mesh I4 isused to control the flow of electrons through the tube. For this purpose, one face of the silica screen I4, is coated with a photosensitive film I6. When mounted Within the envelope II) of the television camera pickup tube,

a light picture or image Il may be focussed through an optical system I9 upon the photosensitive layer I6. Photoemission will take place from the sensitized surface I6 of the silica mesh, in accordance with the amount of light falling on each elemental area of surface I6. Thus, there results on the sensitized film I6, of the silica screen I4, a charge pattern or picture corresponding to the light picture focussed on the screen I4. This charge pattern may be then utilized to control the passage of low velocity electrons therethrough either from a scanning electron beam or from a continuous spray of electrons.

In the tube of Figure 4, there'is used an electron beam I5 for scanning the side of the silica screen I4 which is not coated with-the photosensitive film I6. Electron beam I5 is formed from the thermionic emission from a cathode electrode 20 by the actionof a control electrode 22 and a first anode electrode 24. The electron beam is focussed and accelerated by a second anode electrode, formed as a Wallcoating 26 on.

the inside surface of the tube envelope Ill. The beam I5 is scanned across the surface ofthe fine mesh target I 4 by two pairs of scanning coils respectively 21 and 29, which provide magnetic deflecting fields perpendicular to each other and the axis of the tube envelope Ill. The pairs,

of coils 2'! and 29 are respectively connected to appropriate circuits providing saw-tooth voltages as is well known in the art. The'several electrodes shown in Figure 4 are connected to a D. C. voltage supply 30 to maintain the required potentials on the electrodes during tube operation.

The potentials are such as to form and focusanode electrode 26 is maintained by the voltage supply at close to 1000 volts positive relative to the cathode ground potential. To slow down the electrons of beam l as they approach the target electrode 14, and also to provide an electrostatic field to align the beam parallel with the tube axis as it approaches the target M, an electrode 28 is formed as a wall coating between the second anode coating 26 and the target electrode l4. The potential of electrode 28 is maintained at approximately volts positive relative to cathode ground potential. The electrostatic field produced between the electrodes 26 and 28 tends to bend the electron beam I5 so that it Will approach target [4 in a direction normal to the target surface.

As beam 15 is scanned over the surface of the target l4, the portion of the beam which will penetrate through each interstice of the screen will be an amount proportional to the potential to which the surface of the silica screen surrounding the interstice is raised by the photoemission described above. Thus, there will be provided an electron flow through the fine mesh target H, which will be modulated by the charge pattern formed on the silicate target M by the photoemission from film I5. This modulated stream of electrons is collected by a positive electrode [8 which may be formed as a wall coating on the inner surface of the tube envelope Ill between the target I4 and the end of the tube. Electrode I8 is maintained in the tube of Figure 4 at around 25 volts positive relative to cathode ground potential and also forms the collector of the photoemission from surface IS. A lead 32 connects electrode IS with the grid of amplifier tube 34, as shown, which may be connected in an appropriate amplifying circuit. In this manner, the output of tube 34 provides the video si nal of the pickup tube.

The advantage of using the silica mesh in such an application is the low capacity existing between opposite surfaces of the silica screen. This property enables the photoemission from the sensitized surface [6 to build up relatively high differences of potential between the scanned surface of the screen and the sensitized surface of screen H. A low capacity electrode of this type greatly increases the sensitivity and picture contrast of the video signal of the camera television tube Hi.

It has been shown that insulating screens made in my novel method described above have a better uniformity and a larger portion of open area in the screen than similar screens made by other methods. A silica screen formed by this novel method thus provides a higher transmission of beam current in a pickup tube similar to that shown in Figure 4. For this reason, then, the sensitivity of a pickup tube utilizing a screen made according to my invention is greater than would be possible with an insulating screen formed by other methods.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope or the invention.

What I claim is:

1. The method of forming a silica screen, said method comprising the steps of, coating a metal wire screen with silica by the reaction'of silicon tetrachloride and water vapor, partially drying the silica coating to form cracks through the coating, placing the coated metal wire screen in an acid to remove the metal screen and form a unitary silica screen. '1'

2. The method of forming a silica screen, said method comprising the steps of, chemically precipitating a coating of silica upon a metal screen, removing said metal screen from said silica coating by acid.

3. The method of forming a silica screen, said method comprising the steps of, chemically precipitating a coating of hydrated silica upon a metal screen, partially dehydrating said silica to form cracks through the silica coating, placing the coated metal screen in an acid solution to remove the metal from the silica coating, and dehydrating the silica coating to provide a mesh screen of dehydrated silica. I

4. The method of forming a silica screen, said method comprising the steps of, coating a metal wire screen with silica by the decomposition of ethyl silicate, removing said metal screen from said silica coating by chemical reaction;

5. The method of forming a silica screen, said method comprising the steps of, exposing a metal wire screen to ethyl silicate vapor, decomposing said ethyl silicate vapor to deposit a continuous silica coating on said wire screen, removing said wire screen from said silica coating by chemical decomposition of the metal of said screen.

6. The method of forming a fine mesh silica screen, said method comprising the steps of, chemically precipitating a coating of silica upon the wires of a fine mesh metal screen, "continuing said precipitation of silica until thefwires of said screen are completely coated and the interstices of said metal screen remain open, removing the metal of said screen from within said silica coating.

HAROLD BELL LAW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 533,512 Walsh, Jr. Feb. 5, 1895 2,156,156 Mahlck Apr. 25, 1939 2,188,121 Smith Jan. 23, 1940 2,239,551 Dalton et al. Apr. 22, 1941 2,240,186 Iams Apr. 29, 1941 2,268,589 Heany Jan. 6, 1942 2,272,342 Hyde Feb. 10, 1942 2,275,952 Freeman Mar. 10, 1942 2,540,623 Law Feb. 6, 1951 OTHER REFERENCES Ser. No. 125,892, Wempe (A. P. C.) published Apr. 27, 1943. 

2. THE METHOD OF FORMING A SILICA SCREEN, SAID METHOD COMPRISING THE STEPS OF, CHEMICALLY PRECIPITATING A COATING OF SILICA UPON A METAL SCREEN, REMOVING SAID METAL SCREEN FROM SAID SILICA COATING BY ACID. 